In wireless local area networks (WLAN), an access point (AP) or base station (BS) serving a particular cell broadcasts a beacon signal periodically (e.g., every 100 ms). An active WTRU scans the beacon signal and associates itself with the AP having the strongest beacon. A conventional beacon contains information corresponding to the AP such as a timestamp, beacon interval, capabilities, extended service set (ESS) identification (ID), and traffic indication map (TIM). The WTRU uses the conventional beacon to distinguish between different APs. The WTRU tracks the received signal strength (RSS) of the beacon. When the RSS becomes weak, the WTRU starts to scan for stronger beacons from neighboring APs.
The WLAN conventional scanning process can be either active or passive. In passive scanning, the WTRU simply listens to available beacons. In active scanning, the WTRU sends a probe request to a targeted set of APs that are capable of receiving its probe. In the case of active scanning, each AP that receives the probe responds with a probe response that contains the same information that is available in a conventional beacon with the exception of the TIM. The TIM is used to alert or wake-up APs and is therefore not necessary where an AP is providing a solicited response by responding to a probe request.
In order to facilitate interoperability between WLANs and available alternative systems (i.e. other systems whose coverage areas overlap that of the WLANs), however, it is of interest to allow WTRUs operating within a WLAN to detect the presence of these alternative available systems. One approach for accomplishing this task when handing over from a WLAN to a public land mobile network (PLMN) is to perform a cell search for PLMN availability while simultaneously using the WLAN for communication. This approach may be implemented by having the WTRU's PLMN receiver on and searching for available networks while the WTRU uses the WLAN for communication. The disadvantage of this approach is that it is time consuming and requires the simultaneous activation of a WTRU's WLAN and PLMN functionality. This results in increased complexity and increased battery consumption, and does not permit the WLAN to direct the traffic to a specific PLMN network. Furthermore, this approach is limited to handover from WLANs to PLMNs.
Another approach is where the search for the availability of a specific available PLMN is done manually. With this approach, in addition to being limited to PLMNs, users must manually select one network over the other which is inconvenient and time consuming.
Accordingly, it would be desirable to provide handover information without the disadvantages and limitations of the prior art.